One of the most important and difficult drive train machining operations is the machining of cylinder block head cam bores and cylinder block crank journals--the long, straight interrupted bores that provide support for cams and crankshafts, respectively. Such machining operations that form spaced coaxial bores of common diameter and spaced relatively close together (less than 5 bore diameters) is known in the art as line boring. A series of spaced coaxial bores is sometimes referred to as an interrupted line bore. In high volume automotive applications, forming interrupted line bores such as cylinder block crank bores and cylinder head cam bores is one of the most critical of machining operations. This is because crank journals and cam bores have large length to diameter (L/D) ratios and because tolerances must be held very close to minimize wear between a crank shaft and an engine block or a cam shaft in a cylinder head. Currently, the time required to produce an interrupted line bore of this type is approximately 35 seconds per engine block or cylinder head. This makes the line boring operation one of the most time consuming operations in a serial engine machining and fabricating process. Therefore, it would be highly advantageous to automotive drive train manufacturers if they could reduce this machining time.
Dedicated transfer line technology is currently used to mass produce automotive power train components such as engines and transmissions. Depending on what volume of engines and transmissions must be produced, dedicated transfer lines will typically include 60 to 70 machine stations for cylinder heads and 140 to 150 stations for engine blocks. The machine stations include tooling that is dedicated, i.e., specifically designed to fabricate parts for only a single engine or transmission model. As a result, model changes require extensive re-tooling in the machine stations. The heavy investment required to create or retool a dedicated transfer line can only be justified if annual production volume for a given model will exceed 300,000 units and if that model will be produced for between 10 and 15 years.
The highly competitive international automotive market is now plagued by an over capacity for automobile production. Simultaneously, this customer-driven environment demands that new products be introduced more quickly and in greater variety. In addition, government fuel economy and emission regulations have imposed additional limitations on power train component design and manufacturing. Unfortunately, the current dedicated tooling used in automotive engine production is incapable of supporting the quick introduction of new product designs.
The changes in the international automotive market are creating an urgent need for new power train machining technologies that can provide manufacturing flexibility and upgradability at affordable cost. With the introduction of computer numerically controlled (CNC) machines into power train machining systems, the level of flexibility in engine production has increased considerably in recent years. One exception to this increase in the level of flexibility is in the machining of long bores such as cam and crank journals. Most line-boring operations are still carried out using dedicated manufacturing stations. The dedicated nature of these stations impedes the achievement of full flexibility in the production of drive train components.
The line-boring of cam and crank journals is carried out in predrilled holes or holes in cast, forged or extruded components. The boring process includes using a single point cutting tool, i.e., a boring bar having a single cutting insert, or a boring bar with multiple cutting inserts to remove metal from the pre-existing holes to enlarge or fine finish those holes. This can be accomplished either by rotating the workpiece or by rotating the boring bar. This operation can be performed horizontally, vertically or at some angle between the horizontal and the vertical.
The accuracy and precision achievable in a line boring operation depends on machine structure design, spindle speed, work holding devices, cutting insert and workpiece materials, cutting insert geometry, and the determination and use of optimum cutting speeds and feed rates. Machine movement inaccuracies introduce geometric errors. Excessive thermal loads and their variation introduce errors due to thermal strains.
The size of the holes to be bored dictates tool bar diameter. Generally, when the depth of the boring process is held constant, it is easier to machine larger diameter bores because a larger diameter tool is more rigid and can operate at slower spindle speeds.
Boring bar length is the most critical factor. If the depth of the hole to be bored is large, excessive tool overhang results. Process stability decreases as the length to diameter (L/D) ratio increases. This is because of the resulting vibrations, excessive deflection and loss of stiffness that result from large LID ratios. Problematically, there are many applications that call for boring bars having large L/D ratios while requiring extreme precision. In such cases, the boring bar may be supported by bushings both at the machine tool spindle and at the free end. The boring bar may, additionally or alternatively, be supported by intermediate bushings spaced along the length of the bar. Various boring bar and tool clamping configurations will affect the stability, deflection and vibration encountered during the boring operation. Upon engagement, the tangential force and the radial cutting force will attempt to push the tool away from the workpiece. This results in boring bar deflection. All of these factors must be managed during the boring process to ensure that the process is stable and capable of producing the precision required in a given product.
In a typical dedicated transfer line boring station, the spindle is mounted horizontally and can travel only in the horizontal (feed) direction. Attached to the spindle is a single dedicated boring bar that is configured to machine an interrupted line bore through the entire length of the engine or cylinder head casting. Due to the length of the bore, the boring bar is usually supported, as discussed above, by intermediate support bushings at a number of points along its length and/or outboard bushings adjacent either end of the boring bar to reduce the effective overhang and to ensure high machining precision.
To address the problems presented by line-boring tools having large L/D ratios, a method for minimizing the overhung length of the boring bar or tool has been successfully applied in high volume engine production. According to this method, the spindle is configured to allow a boring bar to be pushed through it. This allows the boring bar to be retracted into the spindle before entering the engine block. As a result, the spindle may be positioned closer to the part and the unsupported length of the bar is thus minimized.
Although this method increases the precision of the long bore machining process;
it also includes a major disadvantage in that the spindle must be made to receive a boring bar of only one specific size. The cost associated with designing and fabricating each such spindle and the dedicated nature of the machine make this method very impractical for low and mid-volume production runs.
Other drawbacks of dedicated transfer line stations include their cost and changeover time. Another drawback is that dedicated tooling of this type can only be used for a specific bore size. In addition, the requirement to use multiple support bushings limits flexibility in the machining operation and creates additional maintenance problems.
The problems associated with dedicated line boring operations can be partially alleviated by using a shorter tool, and mounting either the workpiece or the tool on an index table. The index table allows the shorter tool to form an interrupted line bore by alternately entering a workpiece from opposite sides of the workpiece. This approach decreases the length of the tools by 50% but introduces new inaccuracies related, in part, to the difficulty of precisely aligning and holding the tool in exactly opposite axial positions while forming the bore. Another approach is to double the number of working spindles equipped with tools and to position the spindles and tools so that the tools for an interrupted line bore in a workpiece by entering the workpiece from opposite sides simultaneously. However, this is a very expensive solution.
In machining operations other than line boring, flexibility has been introduced through the use of automatic tool changers and machines that introduce relative motion between spindle and workpiece in two or more axes. For example, U.S. Pat. Nos. 5,321,874 and 5,368,539 issued to Mills et al. (the Mills patents) disclose machining systems including boring stations or machining "cells". Each boring cell includes a spindle mounted for motion in three dimensions on X, Y and Z axes. The machining systems disclosed in these patents also include automatic tool changers.
An approach to increasing manufacturing flexibility in line boring operations is disclosed in Japanese publication JP 6318505 A, published Aug. 1, 1988. This publication discloses a line boring apparatus that includes a numerical control (NC) machine configured to automatically move a spindle into a position to selectively engage one of a plurality of boring bars from a boring bar cradle. The NC machine then adjusts the relative positions of the spindle, four intermediate support bushings and a workpiece to form an interrupted line bore in the workpiece. However, while the disclosed apparatus provides some flexibility in line boring it does not solve the limited flexibility or maintenance problems associated with the use of multiple bushings.
Another approach to increasing manufacturing flexibility in line boring is disclosed in U.S. patent application Ser. No. 08/837,650 (the '650 application) which is assigned to the assignee of the present invention and is incorporated herein by reference. According to the '650 application, a workpiece such as an engine block is located and clamped on a pallet and then brought to a boring station where the pallet is located and clamped on a shuttle. The shuttle moves solely along a straight horizontal path to move the engine block into a work station, locate the engine block during a machining operation, and then transfer the block out of the work station. First and second spindles are supported on respective floor-mounted three-axis drive systems at opposite sides of the shuttle. Tool changers select boring bars from a boring bar magazine and chuck the selected boring bars in the respective spindles. A computer numerical control (CNC) positions the spindles and the block and controls other machining parameters for different engine blocks, causing the spindles to insert their respective boring bars off-center into the crankshaft and camshaft bores, then moving the bars to center and backboring. However, the apparatus requires considerable floor space because the boring bar magazines and the three-axis drive systems are laterally displaced from each other. In addition, the apparatus is unable to accurately machine long bores without outboard ends of the boring bars being inserted into outboard pilot bushings. The requirement for outboard bushings requires additional floor space and limits flexibility because, for each boring bar to be used, an outboard pilot bushing must be pre-positioned adjacent the shuttle in a position opposite the workpiece from the spindle. In addition, for each boring bar to be used, an inboard pilot/support bushing must be pre-positioned adjacent the shuttle.